In rechargeable alkaline cells, weight and portability are important considerations. It is also advantageous for rechargeable alkaline cells to have long operating lives without the necessity of periodic maintenance. Rechargeable alkaline cells may be used as direct replacements for primary AA, C, and D cells in numerous consumer devices such as calculators, portable radios, and cellular phones. They are often configured into a sealed power pack that is designed as an integral part of a specific device. Rechargeable alkaline cells can also be configured as larger cells that can be used, for example, in industrial, aerospace, and electric vehicle applications.
The best rechargeable alkaline cells are ones that can operate as an "install and forget" power source. With the exception of periodic charging, a rechargeable alkaline cell should perform without attention and should not become a limiting factor in the life of the device it powers.
There are two basic types of rechargeable alkaline cells: nickel cadmium ("NiCd") cells and nickel metal hydride ("Ni-MH") cells.
In a NiCd cell, cadmium metal is the active material in the negative electrode. NiCd cells use a positive electrode of nickel hydroxide material. The negative and positive electrodes are spaced apart in the alkaline electrolyte.
Upon application of an electrical potential across the materials of a NiCd cell, the negative electrode undergoes the following reaction: ##STR1## During discharge, this reaction is reversed, Cd is oxidized to Cd(OH).sub.2 and electrons are released. The reactions that take place at the positive electrode of a Ni-Cd cell are also reversible. For example, the reactions at a nickel hydroxide positive electrode in a nickel cadmium cell are: ##STR2##
In general, Ni-MH cells utilize a negative electrode that is capable of the reversible electrochemical storage of hydrogen. Ni-MH cells usually employ a positive electrode of nickel hydroxide material. The negative and positive electrodes are spaced apart in the alkaline electrolyte.
Upon application of an electrical potential across a Ni-MH cell, the Ni-MH material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical generation of hydroxyl ions: ##STR3## The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron.
The reactions that take place at the nickel hydroxide positive electrode of a Ni-MH cell are: ##STR4## This is the identical reaction that occurs in a NiCd cell.
Ni-MH cells can be further classified as V-Ti-Zr-Ni.(Ovonic or AB.sub.2) based or AB.sub.5 (mischmetal) alloys depending on the type of hydrogen storage material used as the negative electrode. Both types of material are discussed in detail in copending U.S. patent application Ser. No. 07/934,976 to Ovshinsky and Fetcenko, the contents of which are incorporated by reference.
The first hydrogen storage alloys to be investigated as battery electrode materials were TiNi and LaNi.sub.5. Many years were spent in studying these simple binary intermetallics because they were known to have the proper hydrogen bond strength for use in electrochemical applications. Despite extensive efforts, however, researchers found these intermetallics to be extremely unstable and of marginal electrochemical value due to a variety of deleterious effects such as slow discharge, oxidation, corrosion, poor kinetics, and poor catalysis. The initial use of these simple alloys for battery applications reflect the traditional bias of battery developers toward the use of single element couples of crystalline materials such as NiCd, NaS, LiMS, ZnBr, NiFe, NiZn, and Pb-acid. In order to improve the electrochemical properties of the binary intermetallics while maintaining the hydrogen storage efficiency, early workers began modifying TiNi and LaNi.sub.5 systems.
The modification of TiNi and LaNi.sub.5 was initiated by Stanford R. Ovshinsky at Energy Conversion Devices (ECD) of Troy, Mich. Ovshinsky and his team at ECD found that reliance on simple, relatively pure compounds was a major shortcoming of the prior art. Prior work had determined that catalytic action depends on surface reactions at sites of irregularities in the crystal structure. Relatively pure compounds were found to have a relatively low density of hydrogen storage sites, and the type of sites available occurred accidently and were not designed into the bulk of the material. Thus, the efficiency of the storage of hydrogen and the subsequent release of hydrogen to form water was determined to be substantially less than that which would be possible if a greater number and variety of active sites were available.
Ovshinsky had previously found that the number of surface sites could be significantly increased by making an amorphous film that resembled the surface of the desired relatively pure materials. As Ovshinsky explained in Principles and Applications of Amorphicity, Structural Change, and Optical Information Encoding, 42 Journal De Physique at C4-1096 (October 1981):
Amorphicity is a generic term referring to lack of X-ray diffraction evidence of long-range periodicity and is not a sufficient description of a material. To understand amorphous materials, there are several important factors to be considered: the type of chemical bonding, the number of bonds generated by the local order, that is its coordination, and the influence of the entire local environment, both chemical and geometrical, upon the resulting varied configurations. Amorphicity is not determined by random packing of atoms viewed as hard spheres nor is the amorphous solid merely a host with atoms imbedded at random. Amorphous materials should be viewed as being composed of an interactive matrix whose electronic configurations are generated by free energy forces and they can be specifically defined by the chemical nature and coordination of the constituent atoms. Utilizing multi-orbital elements and various preparation techniques, one can outwit the normal relaxations that reflect equilibrium conditions and, due to the three-dimensional freedom of the amorphous state, make entirely new types of amorphous materials--chemically modified materials . . . PA1 A disordered semiconductor can exist in several structural states. This structural factor constitutes a new variable with which the physical properties of the material! . . . can be controlled. Furthermore, structural disorder opens up the possibility to prepare in a metastable state new compositions and mixtures that far exceed the limits of thermodynamic equilibrium. Hence, we note the following as a further distinguishing feature. In many disordered materials! . . . it is possible to control the short-range order parameter and thereby achieve drastic changes in the physical properties of these materials, including forcing new coordination numbers for elements . . . PA1 S!hort-range order is not conserved . . . Indeed, when crystalline symmetry is destroyed, it becomes impossible to retain the same short-range order. The reason for this. Is that the short-range order is controlled by the force fields of the electron orbitals therefore the environment must be fundamentally different in corresponding crystalline and amorphous solids. In other words, it is the interaction of the local chemical bonds with their surrounding environment which determines the electrical, chemical, and physical properties of the material, and these can never be the same in amorphous materials as they are in crystalline materials . . . The orbital relationships that can exist in three-dimensional space in amorphous but not crystalline materials are the basis for new geometries, many of which are inherently anti-crystalline in nature. Distortion of bonds and displacement of atoms can be an adequate reason to cause amorphicity in single component materials. But to sufficiently understand the amorphicity, one must understand the three-dimensional relationships inherent in the amorphous state, for it is they which generate internal topology incompatible with the translational symmetry of the crystalline lattice . . . What is important in the amorphous state is the fact that one can make an infinity of materials that do not have any crystalline counterparts, and that even the ones that do are similar primarily in chemical composition. The spatial and energetic relationships of these atoms can be entirely different in the amorphous and crystalline forms, even though their chemical elements can be the same . . .
Once amorphicity was understood as a means of introducing surface sites in a film, it was possible to produce "disorder" that takes into account the entire spectrum of local order effects such as porosity, topology, crystallites, characteristics of sites, and distances between sites. Thus, rather than searching for material modifications that would yield ordered materials having a maximum number of accidently occurring surface irregularities, Ovshinky's team at ECD began constructing "disordered" materials where the desired irregularities were tailor made. See, U.S. Pat. No. 4,623,597, the disclosure of which is incorporated by reference.
The term "disordered," as used herein corresponds to the meaning of the term as used in the literature, such as the following:
S. R. Ovshinsky, The Shape of Disorder, 32 Journal of Non-Crystalline Solids at 22 (1979) (emphasis added).
The "short-range order" of these disordered materials are further explained by Ovshinsky in The Chemical Basis of Amorphicity: Structure and Function, 26:8-9 Rev. Roum. Phys. at 893-903 (1981):
Short-range, or local, order is elaborated on in U.S. Pat. No. 4,520,039 to Ovshinsky, entitled Compositionally Varied Materials and Method for Synthesizing the Materials, the contents of which are incorporated by reference. This patent discusses how disordered materials do not require any periodic local order and how, by using Ovshinsky's techniques, spatial and orientational placement of similar or dissimilar atoms or groups of atoms is possible with such increased precision and control of the local configurations that it is possible to produce qualitatively new phenomena. In addition, this patent discusses that the atoms used need not be restricted to "d band" or "f band" atoms, but can be any atom in which the controlled aspects of the interaction with the local environment plays a significant role physically, electrically, or chemically so as to affect the physical properties and hence the functions of the materials. These techniques result in means of synthesizing new materials which are disordered in several different senses simultaneously.
By forming metal hydride alloys from such disordered materials, Ovshinsky and his team were able to greatly increase the reversible hydrogen storage characteristics required for efficient and economical battery applications, and produce batteries having high density energy storage, efficient reversibility, high electrical efficiency, bulk hydrogen storage without structural change or poisoning, long cycle life, and deep discharge capability.
The improved characteristics of these alloys result from tailoring the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix. Disordered metal hydride alloys have a substantially increased density of catalytically active sites and storage sites compared to conventional ordered materials. These additional sites are responsible for improved efficiency of electrochemical charging/discharging and an increase in electrical energy storage capacity. The nature and number of storage sites can even be designed independently of the catalytically active sites. More specifically, these alloys are tailored to allow storage of hydrogen atoms at bonding strengths within the range of reversibility suitable for use in secondary battery applications.
Based on the pioneering principles described above, a family of extremely efficient electrochemical hydrogen storage materials were formulated. These are the Ti-V-Zr-Ni type active materials such as disclosed in U.S. Pat. No. 4,551,400 ("the '400 Patent") to Sapru, Hong, Fetcenko, and Venkatesan, the disclosure of which is incorporated by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the '400 Patent utilize a generic Ti-V-Ni composition, where at least Ti, V, and Ni are present with at least one or more of Cr, Zr, and Al. The materials of the '400 Patent are generally multiphase materials, which may contain, but are not limited to, one or more phases of Ti-V-Zr-Ni material with C.sub.14 and C.sub.15 type crystal structures. Other Ti-V-Zr-Ni alloys may also be used for a rechargeable hydrogen storage negative electrode. One such family of materials are those described in U.S. Pat. No. 4,728,586 ("the '586 Patent") to Venkatesan, Reichman, and Fetcenko for Enhanced Charge Retention Electrochemical Hydrogen Storage Alloys and an Enhanced Charge Retention Electrochemical Cell, the disclosure of which is incorporated by reference. The '586 Patent describes a specific sub-class of these Ti-V-Ni-Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. The '586 patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them.
The V-Ti-Zr-Ni family of alloys described in the '586 Patent has an inherently higher discharge rate capability than previously described alloys. This is the result of substantially higher surface areas at the metal/electrolyte interface for electrodes made from the V-Ti-Zr-Ni materials. The surface roughness factor (total surface area divided by geometric surface area) of the V-Ti-Zr-Ni is approximately 10,000. This value indicates a very high surface area. The validity of this value is supported by the inherently high rate capability of these materials.
The characteristic surface roughness of the metal electrolyte interface is a result of the disordered nature of the material. Since all of the constituent elements, as well as many alloys and phases of them, are present throughout the metal, they are also represented at the surfaces and at cracks which form in the metal/electrolyte interface. Thus, the characteristic surface roughness is descriptive of the interaction of the physical and chemical properties of the host metals as well as of the alloys and crystallographic phases of the alloys, in an alkaline environment. These microscopic chemical, physical, and crystallographic parameters of the individual phases within the hydrogen storage alloy material are believed to be important in determining its macroscopic electrochemical characteristics.
In addition to the physical nature of its roughened surface, it has been observed that V-Ti-Zr-Ni metal hydride alloys tend to reach a steady state surface condition and particle size. This steady state surface condition is characterized by a relatively high concentration of metallic nickel. These observations are consistent with a relatively high rate of removal through precipitation of the oxides of titanium and zirconium from the surface and a much lower rate of nickel solubilizaton. The resultant surface seems to have a higher concentration of nickel than would be expected from the bulk composition of the negative hydrogen storage electrode. Nickel in the metallic state is electrically conductive and catalytic, imparting these properties to the surface. As a result, the surface of the negative hydrogen storage electrode is more catalytic and conductive than if the surface contained a higher concentration of insulating oxides.
The surface of the negative electrode, which has a conductive and catalytic component--the metallic nickel--appears to interact with chromium alloys in catalyzing various hydride and dehydride reaction steps. To a large extent, many electrode processes, including competing electrode processes, are controlled by the presence of chromium in the hydrogen storage alloy material, as disclosed in the '586 Patent.
In contrast to the V-Ti-Zr-Ni based alloys described above, the early AB.sub.5 alloys are ordered materials that have a different chemistry and microstructure, and exhibit different electrochemical characteristics compared to the V-Ti-Zr-Ni based alloys. However, recent analysis reveals while the early AB.sub.5 alloys may have been ordered materials, more recently developed AB.sub.5 alloys are not. The performance of the early ordered AB.sub.5 materials was poor. However, as the degree of modification increased (that is as the number and amount of elemental modifiers increased) the materials became disordered, and the performance of the AB.sub.5 alloys began to improve significantly. This is due to the disorder contributed by the modifiers as well as their electrical and chemical properties. This evolution of AB.sub.5 type alloys from a specific class of "ordered" materials to the current multicomponent, multiphase "disordered" alloys is shown in the following patents: (i) U.S. Pat. No. 3,874,928; (iii) U.S. Pat. No. 4,107,395; (iv) U.S. Pat. No. 4,107,405; (v) U.S. Pat. No. 4,112,199; (vi) U.S. Pat. No. 4,125,688; (vii) U.S. Pat. No. 4,214,043; (viii) U.S. Pat. No. 4,216,274; (ix) U.S. Pat. No. 4,487,817; (x) U.S. Pat. No. 4,605,603; (xii) U.S. Pat. No. 4,696,873; and (xiii) U.S. Pat. No. 4,699,856. (These references are discussed extensively in U.S. Pat. No. 5,096,667 and this discussion is specifically incorporated by reference.)
Simply stated, in the AB.sub.5 alloys, like the V-Ti-Zr-Ni metal hydride alloys, as the degree of modification increases, the role of the initially ordered base alloy is of minor importance compared to the properties and disorder attributable to the particular modifiers. In addition, analysis of the current multiple component AB.sub.5 alloys indicates that current AB.sub.5 alloy systems are modified following the guidelines established for V-Ti-Zr-Ni based systems. Thus, highly modified AB.sub.5 alloys are identical to V-Ti-Zr-Ni based alloys in that both are disordered materials that are characterized by multiple-components and multiple phases and there no longer exists any significant distinction between these two types of multicomponent, multiphase alloys.
Rechargeable alkaline cells can be either vented cells or sealed cells. During normal operation, a vented cell typically permits venting of gas to relieve excess pressure as part of the normal operating behavior. In contrast, a sealed cell generally does not permit venting on a regular basis. As a result of this difference, the vent assemblies and the amounts of electrolyte in the cell container relative to the electrode geometry both differ significantly.
Vented cells operate in a "flooded condition." The term "flooded condition" means that the electrodes are completely immersed in, covered by, and wetted by the electrolyte. Thus, such cells are sometimes referred to as "flooded cells." A vented cell is typically designed for very low operating pressures of only a few pounds per square inch after which excess pressures are relieved by a vent mechanism.
In contrast, sealed cells are designed to operate in a "starved" electrolyte configuration, that is with a minimum amount of electrolyte to permit gas recombination. The enclosure for a sealed cell is normally metallic and the cell may be designed for operation at up to approximately 100 p.s.i. absolute or higher. Because they are sealed, such cells do not require periodic maintenance.
Typically, a sealed rechargeable alkaline cell uses a cylindrical nickel-plated steel case as the negative terminal and the cell cover as the positive terminal. An insulator separates the positive cover from the negative cell can. The electrodes are wound to form a compact "jelly roll" with the electrodes of opposite polarity isolated from each other by a porous, woven or non-woven separator of nylon or polypropylene, for example. A tab extend from each electrode to create a single current path through which current is distributed to the entire electrode area during charging and discharging. The tab on each electrode is electrically connected to its respective terminal.
In sealed cells, the discharge capacity of a nickel based positive electrode is limited by the amount of electrolyte, the amount of active material, and the charging efficiencies. The charge capacities of a NiCd negative electrode and a Ni-MH negative electrode are both provided in excess, to maintain the optimum capacity and provide overcharge protection.
The operational lifespan, that is, the available number of charge and discharge cycles of a sealed cell, typically determines the kinds of applications for which a cell will be useful. Cells that are capable of undergoing more cycles have more potential applications. Thus, longer lifespan cells are more desirable.
An additional goal in making any type of electrode is to obtain as high an energy density as possible. For small batteries, the volume of a nickel hydroxide positive electrode is more important than weight and the energy density is usually measured in mAh/cc, or an equivalent unit.
At present, sintered, foamed, or pasted nickel hydroxide positive electrodes are used in NiCd and Ni-MH cells. The process of making sintered electrodes is well known in the art. Conventional sintered electrodes normally have an energy density of around 480-500 mAh/cc. In order to achieve significantly higher loading, the current trend has been away from sintered positive electrodes and toward foamed and pasted electrodes that can be manufactured with an energy density of greater than 550 mAh/cc.
In general, sintered positive electrodes are constructed by applying a nickel powder slurry to a nickel-plated steel base followed by sintering at high temperature. This process causes the individual particles of nickel to weld at their points of contact resulting in a porous material that is approximately 80% open volume and 20% solid metal. This sintered material is then impregnated with active material by soaking it in an acidic solution of nickel nitrate, followed by conversion to nickel hydroxide by reaction with sodium hydroxide. After impregnation, the material is subjected to electrochemical formation in alkaline solution to convert the nickel hydroxide to nickel oxyhydroxide.
In all rechargeable cells using a nickel hydroxide positive electrode, the nickel hydroxide changes back and forth between Ni(OH).sub.2 and NiOOH as the cell is charged and discharged. These reactions involve a significant density change during the charge/discharge reactions. This expansion and contraction causes a "swelling" of the electrode. This swelling is a common cause of failure in cells using a nickel hydroxide positive electrode. Failure occurs because as the positive electrode swells, it absorbs free electrolyte from the separator until the separator dries out.
U.S. Pat. No. 5,077,149, describes a cell system to avoid swelling of the positive electrode. The described cell uses a Ni-MH negative electrode, a nickel hydroxide positive electrode, and a sulfonated, non-woven polypropylene separator all of which contain a zinc compound. The zinc compound prevents electrolyte migration to the positive electrode by facilitating electrolyte retention in the negative electrode and the separator. This reduces the expansion of the positive electrode. This patent states that expansion of the positive electrode causes a change in the electrolyte distribution and an increase in internal resistance, and that the use of zinc oxide in the cell, rather than the fabrication of the electrode is the solution to this problem.
Various "poisons", introduced during the production of the positive electrode or generated during the operation of the cell, can also cause cell failure. For example, residual nitrates and Fe are both known poisons.
Residual nitrates occur during impregnation processes that use nickel nitrate. Unfortunately, even parts per million levels of nitrate can result in undesirable self-discharge mechanisms through the formation of the nitrate shuttle reaction.
In both NiCd and Ni-MH cells, free Fe can be leached from insufficiently plated can or tab connections. In addition, some Ni-MH alloys contain Fe, and these materials oxidize and corrode. Once Fe gets into the aqueous electrolyte solution, it is deposited on the nickel hydroxide and reduces the oxygen overvoltage, effectively, poisoning the positive electrode. A reduction in the oxygen overvoltage means that oxygen evolution will occur at the positive electrode before the positive electrode is fully charged, resulting in a reduction in capacity.
It has become standard practice in the Ni-Cd industry to avoid even the smallest Fe impurity in the cell by substituting pure Ni for Fe and by the extensive use of heavy nickel plating. In addition, previously unknown poisoning mechanisms such as deposition or dissolution of metallic species such as oxides of Ti, Zr, or V have been shown to affect the nickel hydroxide electrode in adverse ways such as reduction in capacity, lowered cycle life, and increased self discharge.
In summary, prior art nickel hydroxide positive electrodes have a number of deficiencies that prevent the realization of the full potential of improved Ni-MH negative electrodes. For example, sintered positive electrodes have energy density limitations. In addition, while the use of foamed and pasted electrodes avoid these energy density problems, presently available nickel hydroxide positive electrodes undergo swelling that ultimately results in separator dryout, may have poor rate capability, and are susceptible to poisoning.